1. Field of the Invention
The present invention relates to a dimmable electronic ballast for gas discharge lamps and other devices, and more particularly to a dimmable electronic ballast comprising a loosely coupled transformer, in which current to the lamps is limited by adjusting the frequency of the waveform in the primary winding of the loosely coupled transformer.
2. Description of the Prior Art
An electronic ballast is used to convert commercial electrical power into a high voltage electrical signal sufficient to create and maintain a plasma in a gas discharge lamp. Typical examples of gas discharge lamps are fluorescent lamps and neon lamps. Transformers are typically used in ballast circuits to boost the voltage across the lamps.
Shunted transformers (also generally known as loosely coupled transformers, or leakage transformers) are transformers having higher than usual leakage inductance. Where the intent in standard power transformers is to produce a transformer that is very tightly coupled, that is, to create a transformer with very low reactance, the opposite is true with respect to shunted transformers where the reactance resulting from a shunt is traded off for current limiting capability. The geometry of the transformer core, and the nature of the windings of the coils, are factors that can affect the leakage inductance. All known transformer designs have at least a small amount of reactance, thus the concept of a leakage transformer is based on a relative scale.
As the terms are used herein, a "tightly coupled" transformer is considered to be one in which a very high percentage of the magnetic flux developed in the transformer's primary winding is delivered to its secondary winding. See pages 223, 224, 234, 235 and other general information in Electronic Transformers and Circuits, 2nd Edition, by Reuben Lee, published in 1955 by John Wiley, New York, N.Y. For example, placing the primary winding of a transformer on top of its secondary winding or interleaving the windings will provide a tightly coupled transformer in which substantially all the flux developed in the primary winding "flows" in the secondary winding by physical definition.
A "loosely coupled" transformer, on the other hand, is considered to be a transformer in which a lesser amount of the magnetic flux developed in the transformer's primary winding finds its way to the secondary winding. This relationship can also be expressed in terms of a transformer's coupling ratio, as defined by Lee on page 235 of his aforementioned reference, where "k", the coefficient of coupling (which varies from 0 to 1) is determined from: EQU (I.sub.2.sup.2 Z.sub.2 /E.sub.1 I.sub.1).sub.MAX =k.sup.2 /2(1+(1-k.sup.2).sup.1/2 -k.sup.2 (1)
in which, "I" indicates current, "E" indicates voltage, "Z" indicates impedance, the "1" subscript refers to the primary and the "2" subscript refers to the secondary of a transformer under consideration. Ratio values for "k" below 0.90 are considered to be loosely coupled with ratio values above 0.99 (an arbitrary dividing line) considered to be tightly coupled.
In addition to using magnetic flow as a measure of coupling, it is also possible to ascertain coupling using the inductance exhibited by the primary when the secondary is open and when it is shorted. The condition of the secondary winding circuit, open or shorted, determines the amount of current flow and, derivatively, the inductance exhibited by the primary winding. The ratio of the primary winding inductance under these two extremes gives rise to another form of coupling measurement as shall hereinafter be further described.
Leakage transformers may be gapped or ungapped, depending on the overall design and electrical characteristics. Loosely coupled shunted transformers having an air gap in one of their legs function in the following fashion. Typically, a E-shaped or multi-legged magnetic core is employed with the air gapped leg having a specific magnetic reluctance determined in part by the size of the air gap. One of the non-shunted legs holds the primary winding and the other non-shunted leg holds one or more secondary windings. The shunt is not usually provided with a winding.
The loop including the gapped leg has a fixed reluctance that is significantly higher than that of the secondary loop when the secondary is at low load or is entirely unloaded. In fact, at low load, the secondary winding magnetic loop will have most of the flux flowing through it and the secondary voltage will be high. As the load increases, the reluctance of the secondary loop increases and the secondary voltage decreases. As the secondary load approaches a short or is actually shorted (secondary voltage is zero), the majority of magnetic flux now flows through the gapped leg as its magnetic reluctance is lower than the high reluctance of the secondary winding loop. Thus, at low secondary voltage, the current is high, but limited to a value determined by the reluctance of the gapped leg.
There are basically two types of variable leakage reactance transformers used to control or limit current flow. As described in U.S. Pat. No. 4,123,736 to Brougham entitled LEAKAGE REACTANCE TRANSFORMER, they are commonly referred to as the moving coil type and the moving shunt type. The moving coil type transformer relies on moving one of its windings relative to the other to adjust leakage reactance. In the shunted type transformer, a steel shunt is movably mounted on a frame located between spaced apart primary and secondary windings and is moved into and out of the space between the windings to vary the transformer's reactance. In both types of transformers, degrees of control are predicated on mechanical movement of a winding or a shunt and it is, therefore, difficult to achieve precision current control at a fixed frequency. Further, as noted in the Brougham reference, the costs of such transformers are relatively high, especially when higher cost arrangements are needed to overcome problems presented by wear, jamming and the lack of precision control.
In U.S. Pat. No. 4,187,450 to Chen for HIGH FREQUENCY BALLAST TRANSFORMER, a transformer is described that is particularly useful in conjunction with solid state, high frequency push-pull inverters for supplying power to discharge lamps. The Chen transformer comprises a pair of facing E-shaped core sections disposed adjacent to one another in a mirror image fashion with their corresponding legs aligned but with an air gap provided between the middle, non-touching legs of the core. The transformer is described as being wound in a special fashion to overcome prior art limitations of insufficient ballasting reactance (needed to overcome the negative impedance at startup exhibited by gas discharge lamps) and magnetic leakage. This reference, however, does not teach any method of utilizing frequency or current control to regulate power or signals provided to the secondary of a loosely coupled transformer.
Another air gapped transformer is described in U.S. Pat. No. 4,888,527 to Lindberg for REACTANCE TRANSFORMER CONTROL FOR DISCHARGE DEVICES. In this prior art device for obtaining current limited control of gas discharge lamps, one leg of a three legged transformer is provided with an air gap and fixed reluctance. The transformer's reactance is varied by means of a separate control winding that varies the reluctance of the transformer leg on which it is wound as a function of a variable impedance included in a control circuit used to drive the control winding.
U.S. Pat. No. 5,192,896 to Qin for VARIABLE CHOPPED INPUT DIMMABLE ELECTRONIC BALLAST teaches an output transformer having a loosely coupled primary and secondary winding and a pair of slidable magnetic shunts. The Qin transformer is constructed from a pair of facing E-shaped ferrite cores having an air gap in its center leg. The primary and secondary windings are separated from each other by a pair of shunt housings in which the movable shunts are slidably mounted. By adjusting the position of the shunts, the parameters of the transformer can be adjusted to match the load requirements.
As described above, there were a number of prior art transformer arrangements that sought to take advantage of the inherent characteristics of shunted transformers by varying winding methods or positioning, using slidable shunts and adding control windings to various portions of such transformers. While these attempts at improving the results achieved by control or modification of reactance transformers did achieve better operating results or manufacturing costs, they still failed to yield the degree of precision, low cost, efficiency and versatility required by modem power transferring arrangements.
Co-pending U.S. patent application entitled, "Frequency Controlled, Quick and Soft Start Gas Discharge Lamp Ballast and Method Therefor," Ser. No. 08/982,975, describes an electronic ballast, and is hereby incorporated by reference in its entirety.
The brightness of a gas discharge lamp can be controlled by adjusting the output power of the ballast. Dimmable electronic ballasts typically use pulse width modulation (PWM) to control output power. In a typical PWM circuit, the width of a square wave pulse is adjusted so as to change the total power delivered to the load. It would be undesirable, in many designs, to vary the frequency because many ballast designs have a resonant output stage that helps boost the output voltage. The driving frequency for the output stage, including the transformer, of a PWM circuit is typically held constant to maintain the resonance.
It would therefore be desirable to have an electronic ballast design that is efficient and less expensive than current designs. It would also be desirable to have an electronic ballast design that does not require a resonant circuit in its output stage, so that frequency modulation can be used to control power instead of pulse width modulation. It would further be desirable to use a low cost leakage transformer in an electronic ballast design to control the output current.